The invention relates to a circuit, respectively an apparatus for supplying a load with protection against current surges by means of controlled semiconductors and electromechanical switches placed in series with this load, and in which signals resulting from the measurement of these currents are distributed in a particular order and with a particular delay not only to the control electrodes of the semiconductors but also to a means for electromechanically controlling these switches.
Protective apparatus of the thermal relay type are already known intended to be placed in a supply circuit in series with a static switch module and with a load such as a multiphase motor and in which current measurement means are associated both with electromechanical means for breaking the circuit and with an electronic device capable of emitting disabling signals intended for the static switch module.
Such a protected supply circuit may be reproached for only providing a relatively limited protection, because of the lack of speed accompanying its operation and because of the impossibility of providing an efficient protection of the semiconductors.
Such deficiencies are more particularly felt when it is a question of protecting lines against the appearance of very intense currents which can be observed in the case of short-circuits. However, such a circuit or apparatus has the advantage of providing galvanic isolation of the load, the utility of which is particularly felt when, under normal operating conditions, conduction is provided by semiconductors and the disabling of which or its immunity to parasites it is sometimes difficult to guarantee.
Furthermore, the semiconductors used for providing these functions are usually formed by thyristors or triacs which can only be disabled when the current passes through a substantially zero value, so that the appearance of a fault current at an unfavorable moment in the half period cannot be processed before 10 milliseconds.
Finally, the expensive semiconductors at present available for forming such circuits or the corresponding apparatus are not capable of withstanding overvoltages of about 1000 V without break-downs, which voltages commonly occur in industrial installations fed from networks whose voltage is from 380 V to 440 V and are further subject to their own voltage drops which require efficient and so cumbersome cooling, as soon as the nominal currents reach 50 A.